Automatic gain control circuit



June 14, 1960 2,941,046

A. G. POKRANT AUTOMATIC GAIN CONTROL CIRCUIT Filed Jan. 27, 1958 'fgggMgg if 574.5 gg

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United States Patent O 2,941,046 AUTOMATIC GAIN CONTROL CIRCUIT AdolphG. Pokrant, Forest Hills, Pa., assignor to Westinghouse Air BrakeCompany, 'Wilmerding, Pa., Va corporation of Pennsylvania Filed Jan. 27,195s, ser. Non/11,4411v s claims. (c1. 33o- 22) automatic gain controlcircuit comprising at least two amplifying stages in which control ofthe output signal amplitude is provided by a combination of effects. Thestages are transformer coupled and eachof the stages desirably comprisesa transistor of the three electrode type. A voltage feedback loop isprovided from the last stage to the preceding stage. Rectiers areincluded in the input portions of the last two stages and provide achange in effective impedance with changes in the amplitude of thesignal applied to the transistors. The rectiiers also provide a changein the bias of the transistors dependent on the input signal level tochange theclass of operation of the transistors. Saturation of one ormore of the stages is obtained when the input signal level is abnormallyhigh. The preceding elects are combined in my circuit to provide anexcellent automatic gain control.

Other objects and advantages of my invention will become apparent fromthe following description and the accompanying drawings in which:

Fig. l is a schematic diagram of the automatic. gain control circuit inaccordance with my invention; and; Figs. 2a and 2b are useful indiscussing the operation of the circuit of Fig. l.

I shall rst describe my automatic gain control circuit and shall thenpoint out the novel features thereof in the appended claims.

Referring to Fig. l, the input signal source is coupled throughtransformer 11 havinga primary winding 12 and secondary winding 13 to aP-N-P type transistor 14 consisting of emitter electrode 15, a baseelectrode 16 and a collector electrode L17. One terminal of secondaryWinding 13 is connected directly to the emitter 15, and the otherterminal thereof is connected through a resistance 18 to the positiveterminal of a source of potential 20. Base electrode 1b is connected tothe junctionV point of resistances 21 and 22. The opposite terminal ofresistance 21 is connected to the positive terminal of source 20 and theopposite terminal of resistance 22 is connected through a resistance 23to the negative terminal of source 20. Collector 17 is connected througha current limiting resistance 24, and primary winding 26 of atransformer 25 to the junction of resistances 22 and 23. The output fromtransistor 14 -is coupled by primary and secondary windings-26 and 27 oftransformer 25 to a second amplifying stage consisting of transistor 31.

Transistor 31 includes an emitter electrode 32, a base electrode 33 anda collector electrode 34. One terminal of secondary winding 27 isconnectedto emitter 32Vand.f

the other terminal thereof is connected through rectiiiers 35 and 36 inseries to the positive terminal of source 20. Rectiers 35 and 36 arereverse connected such as to tend to oppose current ow in the emitter32'circuit. Base electrode 33 is connected through resistance 38 to thepositive terminal of source 20. Collector 34 is connected through acurrent limiting resistance 37, primary winding 44 of transformer 43,and resistanceV 42 to the negative terminal of source 20. A bleederorvoltage dii vider resistance 41 is connected from the base 33 to thelower terminal of primary winding 44. The output from transistor31 isYinductively coupled'by primary and sec# ondary windings 44 and 45 oftransformer 43 to a third amplifying Stage consisting of transistor 46.p

Transistor 46 consists of an emitter electrode 47, a base electrode 48and a collector electrode 49. One

terminal of secondary Winding 45 is connected to emitprimary winding 57of an output transformer 58. 'TheV other terminal of the tuned circuit56 is connected through resistance 53 to the negative terminal of source20. A bleeder resistance 52 is connected from base 48 tothe lowerterminal of the tuned circuit 56. The sec' ondary winding 59 oftheoutput transformer 58 is connected to a device such as a relay 61 toactuate desired external circuitry.

Transistor 14 is' biased to operate as a 'class A amplifierV andtransistors. 31 .and 46 "are biased to operate as class B amplilers. InoneV practical embodiment a pulsed carrier waveis to be processed andtransistor 14 is included in the circuit to serve Yas alimiter when theinput4 signal amplitude ,reachesfa predetermined high level., Thusafter.a'predeterminedv signal isv appliedy across transformer 11,transistor 14 will saturate andV clip 'or Vlimit any'uhigher values ofapplied signals.`

QA small amount of current normally o-wsvin the remitter 47 circuit of`transistor46 v'whic'h circuit consists Yof" reverse connectedrejctiers36 andV 51, secondary. Wind.

ing 45 of transformer 43 and emitter47 to lbase 48; The reverseimpedance of the rectifers` increaseswith an increase in the currentflowing therethrough, see Fig. 2b. WithV ano-signal or' small signalinput the impedance across rectiers 36 and 51 will be relativelylowAand,

a large' part of'the signal voltage drop will occuracross theemitter 47to base 48 electrodes. A larger signal causes an increase in the current-flowing in the emitter 47 circuit which in turn will cause an increasedvoltage drop across rectiier 36, see Fig. 2a. As notedY before andas'can'be seen from Fig. 1, rectifier 36 also forms a part of theemitter 32 current path for transistor 31. The vvoltage drop lacrossrectifier 36 biases emitter 32 somewhat higher thandesired `and,transistor 31 would tend to operate as a class C amplifier; AV certainminimum signal wouldv then have to be applied to thecircuit beforetransistor 31 could function as an ampli-lier. A ybias suicientin valueto 'counteract this effect to assure transistor 31 is biased for class Boperation Vand developed across resistance 38. The current ow throughresistance 38 is suflicient to lower the emitter 32 to baseampliers'also increases. As noted above an increase Patented June 14,,1960,

' the i maplin esidependent on`th'e' amount of current flowing Havingthus described my invention, whatl claim is:Y

1. An automatic gain control circuit comprising first and secondtransistors connected in cascade and each having input and outputportions, means connecting a signal voltage to the input portion of saidfirst transistor, iirst, second and third rectiiiers each having a lowforward impedance and alhigh reverse impedance which reverse'impedanceincreases with 'ani' increaselin" reverse op'e'r oliand second,-the totalirnpedancein the Ve'rnitter vcircuit f each' transistor isincreased; Anincrease in crease-j ine pere'efage of increase will beless tirannie percentage increase inV the voltage,developed acrossrectiii'ers'36 and 5I. As a result, the signal input to transis't'r's31aid 46`is controlledv iii inverse relation tothe the increased impedanceVvof recitiifs'iSS," 3G' and Sli which'jnipedace is ftiirfn proportional'incre'aed Signal level also pi' vides iautomatic gainecontr'ol action:

Fui-ther, 'aL degenerative' voltage proportional` to' the" Ysignal levelis -fed back to transistor 311.` lftu willfbesee' that the"'vo1tege'developed across rectifier 36 duer to,

'current' 'new' Vin'`theemitter" 47 ei'feuirt' er transistor" 46fw'1imrfaet ben impressed rheinittef az circuit; of* tran:

new@ .tiene said trarfisistrisl andl circuit is'operble" of preventbeekdewnthereef.;

tlie"VIA aifiplitudes.-

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"investies hslbeesdeseea'fwfth' refejreseeij toa" piticular'erb'odimentthereof, will 'be `understood that" varios'inbdictions" rn'a'y' beinadeby those 'skilled ers being connected to ,the inputportion of saidsecond transistor to presentah'igh reveisewimpedance to current iiowthroughsaid second transistor, each of said transistors and therectiers' connected respectively thereto eiecting adivision ofthe signalvoltage impressed thereacross .tovary the percentage' of theV signalvoltage developed across each of said transistors inversely with anyvariation in said signal voltage. f '5 2. An automatic gain controlcircuit comprisingy first' and second transistors each having input andoutput Vportions, means connecting an operatingpotential' to saidtransistors-meansY inductively coupling a signalvoltage to'th'e' inputportion Yof said first transistor, the output portion of said iirsttransistor beingjinductively coupled to'. the input portion of saidsecond transistor, first', sec ond'andthird rectifying" elements Veachhaving a low forward` impedance and a high reverse impedance whichreverse impedance increases'with an increase` in reverse currentowingth'erethrough, means connectingsaid rectifyingy elements-to acommon junction, saidiirst and second rectifyingzelements beingkconnected to the input portion of sa'id'iirst Ytransistor to present ahigh 'reverse impedance4 to'currentY flowing lthrough said firsttransistor, said lirst and said third Irectifying; elements b eingconnected-to theinput portion ofsaid second transistor to presentia'highreverse impedance to current iiow through saidjseconl transistorwhereby-said rectiiiersdeyelop a voltage'therea'cross which varies ,withthe signalvoltagej Ylector electrodes, means connecting anoperatinglpotcntial`^to: 'saidll transistors, transf'orm'ermeansinductively kctmplingwaV sgnal'voltage to the. emitt er of said rsttransistor,` transiormer, k.means -finductivelyj coupling the collectorof said iirst transistor to theemitter of saidsec- Y ond transistor;first, second and third rectifying elements each-having alow forwardimpedance and a highreverse impedance which reverse impedance,increaseswith an increase in' reverse fcurrent ilowing therethrough',means connecting' said 'rectifying4 elements kto a commonjunction,--said iirst and secondnrectifyin'g elements being connected'inseries with the emitter 4of said iirst transistor to; vpresent ay high'Areverse impedance rto currentr flowing through'said iirst transistor,said irst and said third recti fying'elements being vconnected in serieswith the emitter ofl said 'second transistor to present a high Mreverseim- Vpedancefto currentjiiow through said second transistorj wherebysaid rectiiiers develop` a Vvoltage thereacross which varies with thesignal voltage to bias 'the transistors connectedgin` series therewithto oppose changes in current o'w therethrough-.

4." An automatic; gaincontrol circuit comprising a pair ofca'sc'adecoupled transistors eachhavingfinput and in-the artwithoutdparting'ffroin theVV invention; Thev otputportions,Imeans'coupling-'a signal voltage to the input portionofthe irs't'ofsaidtransistors, first, second and thirdV rectiiiers' "ech'having a lowforward impedance 'and a high reverse'impfedance whichV increases with'an increase in reversev currentp-o'win'gA therethroigh, Vmeans coy'ectin "said rectiiiersy to'acommonfjunction, meanscciiinTectin'gast/riff"4 first3 andi-'second rectiiiers""'to'fthe'inputf portion of said iirst transistor to present a high reverseimpedance to current ow through said inst transistor, means connectingsaid rst and third rectiiiers to the input portion or the second of saidtransistors to present a high reverse impedance to current ow throughsaid second transistor, the input po n of said first transistor and saidirst and second rectmers effecting a division of the signal voltageimpressed thereacross to vary the percentage of the signal voltagedeveloped across said rst transistor inversely with any variation insaid input signal voltage, the input portion of said second transistorand said first and third rectiiiers effecting a division of the signalvoltage coupled from said rst transistor to said second transistor tovary the percentage of the signal voltage developed across said secondtransistor inversely with any variation in 'signal voltage coupledthereto, and said common junction connecting the voltage developedacross said irst rectier due to current ow in said second transistor asa feedback voltage to said rst transistor.

5. An automatic gain control circuit comprising first and secondtransistors each having emitter, base and collector electrodes, meansconnecting an operating potential to said electrodes, transformer meanscoupling a signal voltage to the emitter of said iii-st transistor,transformer means coupling signal voltage from the collector of saidfirst transistor to the emitter of said second transistor, iirst, secondand third rectiers each having a low forward impedance and a highreverse impedance which reverse impedance increases with an increase inreverse current owing therethrough, means connecting said reetiers to acommon junction, means connecting said rst and second rectiers in serieswith the emitter of said rst transistor to present a high reverseimpedance to current ow through said rst transistor, means connectingsaid first and third rectiers in series with the emitter of said secondtransistor to present a high reverse impedance to current ow throughsaid second transistor, the emitter to base path of said rst transistor,and said first and second rectiers effecting a division of the signalvoltage impressed thereacross to vary the percentage of the signalvoltage developed across said first transistor inversely with anyvariation in said signal voltage, the emitter to base path of saidsecond transistor and said rst and third rectiers effecting a divisionof the signal voltage coupled from said first transistor to said secondtransistor to vary the percentage of the signal voltage developed acrosssaid second transistor inversely with any variation in signal voltagecoupled thereto, and said common junction conneeting the voltagedeveloped across said first rectiier due to current ow in said secondtransistor as an inverse feedback voltage to said rst transistor.

References Cited in the le of this patent UNITED STATES PATENTS2,833,870 Wilhelmsen May 6, 1958

